1. Field of the Invention
The present invention relates to an auto tensioner applying a tensile force to a belt or a chain which is provided in a tensioned state between a drive wheel and a driven wheel.
2. Description of the Related Art
In order to apply the tensile force to the belt or the chain (hereinafter, referred to as the belt), when the tensile force of the belt is suddenly increased and a plunger is going to be compressed, the auto tensioner generates a damping force against the compression. When the tensile force of the belt is suddenly reduced and the plunger is going to be expanded, the auto tensioner acts to expedite the expansion.
Further, there is a hybrid vehicle structured such that two driving sources comprising an engine and an electric motor are served by one belt, whereby the driving sources are appropriately changed. In the hybrid vehicle mentioned above, there is a structure in which an auto tensioner adjusting a tensile force of the belt is arranged at a position in a downstream side of the engine and an upstream side of the electric motor. However, in the case that the engine is started by the electric motor under the engine stop state, the auto tensioner operates so as to compress, and slipping is generated in the belt. Accordingly, in order to inhibit the belt from being slipped, it is necessary to lock a compression stroke of the auto tensioner.
However, in the conventional auto tensioner (Japanese Patent No. 2537547), since a damping force in a pressure side is generated by circulating a working fluid in a high pressure oil chamber through an annular gap between an inner periphery of a cylinder and a plunger, it is impossible to close the annular gap. Accordingly, it is difficult to provide the lock mechanism of the compression stroke.
Further, in the other conventional auto tensioner (Japanese Patent Application Laid-open No. 1-106924), in order to hold a belt transmission under a preload based on operation of an electromagnetic valve during reverse rotation of the engine, a shock absorber for locking a compression stroke of a piston rod is employed. However, in the auto tensioner, the piston rod is fixed in a position when the shock absorber is compression locked, so that the auto tensioner cannot expand the piston rod so as to absorb slack of the belt, when the belt is further loosened.
In this case, in Japanese Patent Application No. 2002-318933, the applicant of the present invention has proposed an auto tensioner in which a plunger is slidably inserted into a cylinder. The cylinder is mounted to a stationary member side positioned in an upper side. The plunger is mounted to a pulley side positioned in a lower side. A high pressure oil chamber sectioned by the cylinder and a pressure applying portion of the plunger is formed within the cylinder. An outer tube sectioning an oil reservoir chamber having a gas chamber in an upper portion is provided in an outer periphery of the plunger and the cylinder. A first oil passage communicating the gas chamber with the high pressure oil chamber is provided in an upper portion of the cylinder. A second oil passage communicating the high pressure oil chamber with the oil reservoir chamber is provided in the plunger. The first oil passage is provided with an orifice hole and a first check valve performing a closing operation during expansion, and the second oil passage is provided with a second check valve performing a closing operation during compression.
However, according to the auto tensioner proposed by the applicant of the present invention, since the orifice hole provided in the first oil passage in the upper portion of the cylinder so as to structure the pressure side damping force generating means is positioned in the upper side with respect to an oil surface of the high pressure oil chamber, a delay or insufficiency is generated in sucking the working fluid to the high pressure oil chamber, so that air exists in the upper portion of the high pressure oil chamber. It is impossible to sufficiently damp a tensile force fluctuation in a high frequency area of the belt.